Instead of steel wire ropes that have been used successfully on cranes for many years, it is recently being tried to use high-strength fibre ropes made of synthetic fibres such as aramid fibres (HMPA), aramid/carbon composites, highly modular polyethylene fibres (HMPE) or poly(p-phenylene.2,6-benzobisoxazole) fibres (PBO). The advantage of such high-strength fibre ropes is their low weight. At equal rope diameters and equal or higher tensile strengths, such high-strength fibre ropes are clearly lighter in weight than comparable steel wire ropes. In particular for high cranes with accordingly long rope lengths, this results in a major weight reduction reflected in the dead weight of the crane leading to higher payloads for an otherwise unchanged crane design.
However, a disadvantage of such high-strength fibre ropes is their breaking behaviour, i.e. their failure without any distinct long-term prior warning. While wear is clearly indicated with steel wire ropes, showing failure long in advance, for example when individual steel wires break and splice open, which is easily detected, high-strength fibres show few signs of excess splicing that could be detected with the naked eye and that would show long before their actual failure. They therefore require intelligent monitoring measures to allow the early detection of when the discard state of high-strength fibre ropes will occur.
It is known from WO 2012/100938 A1 to detect the discard state of a high-strength fibre rope by testing various rope discard criteria which change over the time in which a rope is used and under stress. Here, the rope diameter, the shear stress stiffness measured by the cross-sectional changes resulting when the rope is pinched, and by the number of completed stress cycles. However, the informative value of these individual discard criteria is limited, which means that the interaction of these discard criteria must be monitored and evaluated in a rather complex monitoring process before the discard state can actually be detected with reliability.
Based on this, it is the object of the present invention to provide an improved device for detecting the discard state of high-strength fibre ropes which avoids the disadvantages of the prior art and advantageously develops it further. Preferably, a simple but reliable and precise detection of the discard state is to be achieved which economically utilizes the remaining service life if the fibre rope without jeopardizing safety, and which can be used on construction machinery with simple detection means functioning reliably even under heavy-duty working conditions.
According to the invention, the above object is achieved with a device and a crane. Preferred embodiments of the invention are the subject of the sub-claims.
It is therefore suggested to monitor the rope's bending stiffness and to determine the discard state by means of the rope's bending stiffness. According to the invention, the evaluation unit comprises bending stiffness determination means for determining the rope's bending stiffness, whereby the evaluation unit provides the discard signal depending on the determined bending stiffness of the rope. While steel wire ropes do not show significant changes in bending stiffness depending on the rope's service life, this is different with high-strength fibre ropes. The filaments which are still flexible at the beginning of the rope's use, are made harder and the rope is made stiffer by the tensile stress and the bending stress. This increase in the rope's bending stiffness is easy to measure, which means that the discard state can be determined reliably and precisely by the rope's monitored bending stiffness. It shows that rope twisting tests with a new rope show a rather low bending stiffness while ropes driven to the breaking point show a very high bending stiffness in the end due to prolonged and severe stress, namely many times that of the rope's original state. This increase rises continuously with the cycles-to-failure rate, reaching the highest point when the rope breaks, which means that the evaluation unit can determine the discard state relatively easily.
In the further development of the invention, the bending stiffness determination means can comprise two rope support elements spaced apart from each other and at least one shear force stamp for applying pressure to the rope with a shear force, whereby the shear force stamp and/or the rope support elements can be moved across the lengthwise direction of the rope such that the rope is subjected to curvature. Advantageously, a laterally movable shear force stamp can be arranged between the two rope support elements spaced apart from each other, and essentially across a connecting line through the two rope support elements, whereby the movability can be such that the pressure head or engagement head of the shear force stamp can be moved toward the said connecting line and advantageously across this connecting line. In principle it would also be possible to arrange the said shear force stamp not between the two rope support elements but on a side of the two rope support elements, especially in the area of an extension of the said connecting line beyond the two rope support elements, such that the shear force stamp acts upon the rope like a projecting flexure beam.
In the above mentioned arrangement of the shear force stamp between the two rope support elements, the arrangement is advantageously such that the two rope support elements are arranged on one side of the rope while the shear force stamp is arranged on the opposite side of the rope.
In a further development of the invention, the shear force stamp and/or the rope support elements can be provided with a dynamometer and/or a travel meter for measuring the shear force and/or the travel of the shear force stamp and/or the rope support elements applied across the lengthwise direction of the rope to be tested. Instead of such a travel meter, a deflection sensor could be provided which measures the deflection or displacement of the rope across the lengthwise direction of the rope.
The rope's bending stiffness can be determined with the bending stiffness determination means by means of the deflection of the rope that can be achieved with a predetermined shear force and/or with the shear force required for a predetermined deflection. In a further development of the invention, these two determination criteria can also be used in combination with each other, in particular such that it is determined what force is required for a predetermined deflection and what deflection occurs at a predetermined shear force, thus taking into account any non-linearities that may occur with regard to shear stress stiffness.
In an advantageous further development of the invention, the rope is only supported by the said rope support elements and/or by the shear force stamp without absorbing bending moments or torques that might occur. In particular, the rope support elements and the shear force stamp are designed such that no moment resistance is set against the twisting or bending of the rope. For example, the rope support elements and the shear force stamp can form unilateral supporting points or planes which essentially absorb forces only across the rope's lengthwise direction but do not transfer any bending moments to the rope.
Not to falsify the measuring of the rope's bending stiffness by stresses acting upon the rope from tensile forces, the bending stiffness determination means comprises a tensile force adjuster which always establishes the same tensile force conditions on the rope for repetitive bending stiffness measurements. In particular, the said tensile force adjuster can comprise a tensile force release means which essentially completely releases the rope of tensile forces when the rope's bending stiffness is determined.
In principle, the said tensile force release means can have different designs. In an advantageous further development of the invention, the tensile force release means can comprise holding means for holding the rope in lengthwise direction, preferably at least one rope clamp to clamp the rope, in particular to absorb hoist loads at the lifting hook, and which releases the rope section to be tested for the rope's bending stiffness. In particular, the said rope clamp can be associated with the rope on a side of the bending stiffness determination means facing away from the rope drum, such that when the rope is pinched, a nearly complete tensile stress release can be achieved for the rope section to be tested by releasing the clamped rope or by unwinding the rope drum. Corresponding control means can control the rope drum to unwind it for a predetermined piece or to activate it in the direction of lowering the load, such that rope slack is produced between the rope clamp and the rope drum.
In principle, the evaluation unit for providing a discard signal can work in various ways, for example by monitoring changes in the rope's bending stiffness and/or by monitoring the absolute bending stiffness. In particular, the said evaluation unit can be designed such that a discard signal is provided when the rope's bending stiffness and/or its change exceeds a certain threshold value.
For example, one or more reference measurements can be conducted on a new rope such that the percentage change in the rope's bending stiffness that occurs during operation can be compared with a threshold value for change or that the discard signal is provided when this threshold value is exceeded or reached. In particular, the discard signal can be provided when the rope's bending stiffness rises above a still tolerable threshold value. As an alternative or in addition, the monitored bending stiffness which is constantly or periodically determined during operation can be compared with an absolute threshold value that is provided by the manufacturer for a certain type of rope or for a specific rope, and that the discard signal is then provided when this threshold value is exceeded. Also as an alternative or in addition, the discard signal can be provided when the measured change in the rope's bending stiffness is too rapid and/or too slow, i.e. when the change frequency of bending stiffness exceeds or falls below a threshold value. The speed of change in time can be the speed of change in the number of load cycles which, for example, can be detected with a load cycle counter and considered by the evaluation unit. As an alternative or in addition, the speed of change can also only be taken into account by the number of measurements of the rope's bending stiffness, for example by providing a discard signal when the change in the rope's bending stiffness detected after a certain number of measurements, for example after the tenth measurement, exceeds the threshold value predetermined for that purpose.
The discard signal can simply be indicated to the crane operator, for example acoustically and/or visually, or it can be used to stop the rope drive.
In an advantageous further development of the invention, the bending stiffness determination means can be firmly installed in the rope drive of the hoist such that the rope's bending stiffness can be constantly monitored during operation, i.e. in the operational state of the hoist, without the necessity of having to convert the hoist into a special test modus. As an alternative or in addition, the bending stiffness determination means can also be provided as a detachable unit that can be used in different hoists.
In an advantageous further development of the invention, the bending stiffness determination means are arranged in a rope section of the rope drive which is subject to most bending changes. In the hoisting rope of a tower crane, for example, this can be a rope section intended to run around the deflection pulleys on the trolley and the deflection pulleys on the lifting hook. Depending on the design of the hoist and the course or reeving system of the rope, these can be various rope sections.